ARINC 717

ARINC protocols are used in aircraft to interconnect system components in different parts of the aircraft. The ARINC 717 standard describes the data communication between DFDAU (Digital Flight Data Acquisition Unit), a computer that collects data from various systems and converts it into the ARINC 717 format, and the DFDR (Digital Flight Data Recorder).

Basic idea

The ARINC 717 was introduced, in contrast to the ARINC 429, to achieve a higher level of data security. A direct current-free bi-phase signal with +/- 5V is used. The coding process is called Harvard II Biphase. A logical ZERO holds the voltage value over the entire bit width, with logical ONE the voltage value is switched within the bit width.

Working method

The DFDAU (Digital Flight Data Acquisition Unit) receives status information in the form of individual data elements ( parameters ) from various units in the aircraft . Each element contains an 8-bit identifier for identification and 18-19 bit user data (see ARINC 429 ). Such a data element can describe, for example, the current altitude, airspeed or the position of the rudders . The DFDAU buffers this data and converts it into frames and subframes. A frame contains data describing the 4 second life span of the aircraft. The frame is divided into 4 subframes, each containing one second of the aircraft's lifetime. A subframe contains the data collected by the DFDAU, the amount of data (parameters) depends on the type certification of the aircraft. In older systems 64 parameters were recorded, since the introduction of digital flight recorders 128 parameters. In the USA , 256 parameters are to be recorded in accordance with the requirements of the FAA. On the Airbus A380 , as many as 1024 parameters are recorded.

The individual subframes each contain a set of parameters for one second. The 12-bit user data is saved for each parameter, the identification and synchronization bits are no longer required, because the subframe always has the same structure, so that the type of element can be determined based on the position in the frame. With 64 parameters, the subframe contains 768 bits, which are transmitted one after the other without pauses or other separation. The data rate is therefore 768 bits / second. The next subframe then follows. The subframes differ in the first parameter, which is used for identification (subframe 1, 2, 3 or 4). The remaining parameters are built in according to a pattern-dependent scheme and can only be deciphered with a corresponding table. Not every parameter is transmitted in every subframe. Less important ones are only transmitted in every 2nd or 4th subframe. After a frame (4 seconds) has elapsed, the entire parameter set has been transmitted and the process begins again.

In principle, at least 25 hours of aircraft life are recorded.